Meaning & Usage
When you want to express a firm personal decision, ことにする (koto ni suru) is the ideal grammar point. While the standard ~ます (masu) form simply states a future action, this structure signals that you have reached a conclusion after some thought. It transitions your speech from a simple statement of fact to an expression of personal willpower.
This grammar combines こと (koto), meaning "matter" or "thing," with the verb する (suru), meaning "to do." Literally, you are "doing the matter of [an action]." It implies that among several options, you have consciously selected one specific path. This is why it is frequently used for New Year's resolutions, health goals, or picking an item from a menu.
The distinction between 行きます (ikimasu - I will go) and 行くことにします (iku koto ni shimasu - I have decided to go) is subtle but important. The latter suggests a deliberate choice. You will use this form in both formal and informal contexts. In polite speech, use ことにします or ことにしました. With friends, the casual ことにした is more appropriate.
A common variation is ことにしている. This indicates that a past decision has become a present habit or a self-imposed rule. For example, you might use it to describe a daily exercise routine or a personal policy of not eating sweets after 8 PM.
Structure & Formation
Forming this structure is straightforward. It always attaches to the plain form of a verb. Use the dictionary form for a decision to do something, and the ない (nai) form for a decision to avoid something.
Review the table below for the connection patterns:
| Verb Type | Plain Form | Grammar Attachment | Meaning |
|---|---|---|---|
| Dictionary (Positive) | 食べる | 食べることにする | Decide to eat |
| Nai-form (Negative) | 食べない | 食べないことにする | Decide not to eat |
| Dictionary (Positive) | 買う | 買うことにする | Decide to buy |
| Nai-form (Negative) | 買わない | 買わないことにする | Decide not to buy |
Remember that the verb preceding こと must remain in the plain form. Even in formal sentences, you would never say "tabemasu koto ni shimasu." Politeness is only expressed at the very end of the sentence by conjugating する.
Example Sentences
The following examples demonstrate how this grammar appears in everyday Japanese. Pay attention to how the past tense しました is used for decisions already finalized.
Health & Lifestyle Resolutions
健康のために、タバコをやめることにしました。
kenkou no tame ni, tabako o yameru koto ni shimashita.
For my health, I have decided to quit smoking.
来週から、毎朝５時に起きることにします。
raishuu kara, maiasa go-ji ni okiru koto ni shimasu.
I've decided that from next week, I will wake up at 5 AM every morning.
お金をためたいので、あまり買い物をしないことにしました。
okane o tametai node, amari kaimono o shinai koto ni shimashita.
Because I want to save money, I've decided not to do much shopping.
Travel & Logistics
冬休みは、北海道に行くことにしました。
fuyuyasumi wa, hokkaido ni iku koto ni shimashita.
I have decided to go to Hokkaido for the winter break.
新幹線は高いので、バスで行くことにしました。
shinkansen wa takai node, basu de iku koto ni shimashita.
Since the Shinkansen is expensive, I decided to take the bus instead.
Education & Career
将来のために、毎日一時間日本語を勉強することにしています。
shourai no tame ni, mainichi ichijikan nihongo o benkyou suru koto ni shite imasu.
For my future, I make it a rule to study Japanese for one hour every day.
難しいけれど、今年はＮ４を受けることにしました。
muzukashii keredo, kotoshi wa N4 o ukeru koto ni shimashita.
It is a challenge, but I have decided to take the N4 exam this year.
大学では、経済を専攻することにしました。
daigaku de wa, keizai o senkou suru koto ni shimashita.
I have decided to major in economics at university.
Common Mistakes
Mistake 1: Confusing Choice with Circumstance
❌ 来月から日本へ出張することにしました。
✅ 来月から日本へ出張することになりました。
ことにする implies the decision was yours. However, business trips are usually decided by your employer. In these cases, use ことになる (koto ni naru), which means "it has been decided by external factors." Only use ことにする when you have the authority to make the choice.
Mistake 2: Using Masu-form before Koto
❌ 毎日走りますことにしました。
✅ 毎日走ることにしました。
You must use the dictionary form (run) or the negative form (don't run) before こと. Politeness is managed by changing する to します at the end of the sentence.
Mistake 3: Misapplying to Nouns
❌ 昼ごはんはラーメンことにしました。
✅ 昼ごはんはラーメンにしました。
When choosing a noun directly (like an item from a menu), simply use Noun + にする. The grammar ことにする is strictly for actions (verbs). If you want to include the verb, say ラーメンを食べることにしました.
Cultural Notes
Using ことにした can sound very resolute. In Japanese culture, declaring a decision this way shows a strong personal commitment. When someone says they have decided to quit a habit, it implies a level of self-discipline that is highly respected.
Interestingly, Japanese speakers sometimes use ことになる (it has been decided) even for personal plans. They do this to sound humble and avoid sounding too "ego-driven." By phrasing a plan as if it were decided by circumstances, the speaker appears more modest. However, among friends, ことにした remains the most natural way to share your intentions.
Related Grammar Points
- Bakari (ばかり) — Just Did, Nothing But (Grammar N4)
- ように (You ni) — So That, In Order To (Grammar N4)
- Understanding はずだ: Expressing Logical Expectations (Grammar N4)
- Mitai da: Expressing Similarities and Guesses (Grammar N4)
- Hearsay (I heard that): Souda (Grammar N4)
- Sequence and Starting Points: Using てから (te kara) (Grammar N4)
JLPT Tips
The JLPT often tests the difference between ことにする (subjective choice) and ことになる (objective result). If the sentence mentions a company policy, a wedding date, or a law, the answer is almost always naru. If it involves a personal hobby or diet, look for suru.
Also, keep an eye on the tense. Use ことにした for decisions that are already "on the books." Use ことにする for the moment you are actually choosing between options. In the reading section, you may also see ことに決めました (koto ni kimemashita), which is a more formal version of the same idea.